The present invention relates to development of presensitized plates for use in making lithographic printing plates (hereinafter referred to as "PS plates"), in which aluminum plates are used as substrates, in particular to a method for developing such PS plates suitable for applying to an automatic developing machine.
A positive-working PS plate which has widely been used comprises an aluminum plate serving as a substrate provided thereon with a light-sensitive layer which comprises an o-quinonediazide compound. It is known that the o-quinonediazide compound is converted into a carboxylic acid through irradiation with ultraviolet rays. Correspondingly, if such an exposed PS plate is developed with an alkaline aqueous solution, only the exposed portion on the light-sensitive layer is removed and thus the surface of the substrate is revealed. Since the surface of the aluminum substrate is hydrophilic, the surface portion (non-image area) of the aluminum substrate revealed through the development receives water but repels oil-based inks. On the other hand, the remaining portion (image area) which has the light-sensitive layer even after the development is lipophilic and, therefore, receives inks but repels water.
There have been known a variety of alkaline aqueous solutions used for developing such imagewise exposed positive-working PS plates, but most preferred are aqueous solutions of silicates such as sodium silicate and potassium silicate. This is because, the developing ability of the solution can be adjusted to some extent by controlling the ratio of silicon oxide SiO.sub.2 to alkali metal oxide M.sub.2 O (in general, expressed in terms of the molar ratio, [SiO.sub.2 ]/[M.sub.2 O], in the silicate and the concentration thereof.
These silicates are likewise used as developers for reversal type negative-working PS plates whose light-sensitive layer comprises an o-quinonediazide compound as disclosed in Japanese Patent Publication for Opposition Purpose (hereinafter referred to as "J.P. KOKOKU") No. Sho 56-14970(=U.S. Pat. No. 4,196,003); negative-working PS plates whose light-sensitive layer comprises an alkali-soluble diazonium salt as disclosed in EP-A-0415422; and negative-working PS plates whose light-sensitive layer comprises a resin carrying dimethylmaleimido groups on the side chains as a photocrosslinking agent as disclosed in U.S. Pat. No. 4,416,975 and EP-A-0443,742, in addition to the foregoing positive-working PS plates.
Recently, there have widely been used automatic developing machines in the fields of plate-making . printing industries for rationalizing and standardizing the plate-making operations. The automatic developing machine in general comprises a device for conveying PS plates, a bath for developer and a spraying device and develops an exposed PS plate by pumping up a developer and spraying it on the surface of the PS plate, which is horizontally conveyed, through a spray nozzle. Alternatively, there has also been known a method in which a PS plate is conveyed through a developer contained in a bath by the action of, for instance, a guide roll immersed in the developer to thus develop the PS plate.
Japanese Unexamined Patent Publication (hereinafter referred to as "J.P. KOKAI") NO. Sho 54-62004 (=U.S. Pat. No. 4,259,434) discloses that when positive-working PS plates are treated with such an automatic developing machine, a large quantity of these positive-working PS plates can be treated without exchanging a developer in a tank over a long time period through the use of an aqueous solution of sodium silicate whose molar ratio: SiO.sub.2 /Na.sub.2 O ranges from 1.0 to 1.5 (i.e., [SiO.sub.2)/[Na.sub.2 O] ranges from 1.0 to 1.5) and whose SiO.sub.2 content ranges from 1 to 4% by weight and through continuous or intermittent addition, to the developer, of an aqueous solution of sodium silicate (replenisher) having a molar ratio, SiO.sub.2 /Na.sub.2 O ranging from 0.5 to 1.5 (i.e., [SiO.sub.2 ]/[Na.sub.2 O] ranging from 0.5 to 1.5) depending on the quantity of the positive-working PS plates to be processed.
However, if a large quantity of PS plates are processed by this developing method, insolubles are formed in the developer during processing, adhere onto the processed PS plates and become a cause of clogging of nozzles and of loading of filters. It is known that the formation of such insolubles is conspicuous, in particular, in the development of positive-working PS plates in which an aluminum plate having an anodized layer is used as a substrate. In order to solve this problem, J.P. KOKOKU No. Sho 57-7427(=U.S. Pat. No. 4,259,434) discloses a method in which a developer is used which comprises an alkali metal silicate having [SiO.sub.2 ]/[M] ranging from 0.5 to 0.75 (i.e., [SiO.sub.2 ]/[M.sub.2 O] ranging from 1.0 to 1.5) and an SiO.sub.2 content of 1 to 4% by weight, an alkali metal silicate used in a replenisher has [SiO.sub.2 ]/[M] ranging from 0.25 to 0.75 (i.e., [SiO.sub.2 ]/[M.sub.2 O] ranging from 0.5 to 1.5), and both developer and replenisher comprise at least 20% of potassium on the basis of the total amount (g atom) of the alkali metals present therein. This method can inhibit the formation of insolubles, but the activity of the replenisher is relatively low and this increases the amount thereof to be supplemented. The technique disclosed in J.P. KOKAI No. Hei 2-3065(=EP-A-0347245) can eliminate this drawback, but there has still strongly been required for the reduction in the running cost and the amount of waste liquor. More specifically, there has recently been discussed, on a worldwide scale, about the conservation of natural environment and correspondingly there has been required for the reduction of the quantity of industrial waste.